Archaeological dating by re-firing ancient pots

Researchers in the UK have created a new way of dating archaeological artefacts that involves heating ancient pots to unlock their internal clocks. The relatively simple technique could become as important for dating ceramics as carbon dating is for organic materials, say the researchers at the Universities of Manchester and Edinburgh. The team has already dated ceramics from the Roman, medieval and modern periods to a high degree of accuracy, and they are now looking to establish a global research facility for the technique.

The method relies on the fact that fired clay ceramics — like bricks, tile and pottery — start to chemically combine with water as soon as they are exposed to the atmosphere. A big breakthrough came in 2003 when the researchers realized that this process has occurred at a predictable rate throughout history, related to temperatures. Now the researchers have turned their theory into a practical dating method and present their findings in Proceedings of the Royal Society A.

Moira Wilson of the University of Manchester and her team document how “rehydroxlation dating” has so far dated objects up to 2000 years old, and they believe it could extend back as far as 10, 000 years. “Given the number and intensity of [dating] debates in archaeology, there is a huge gap in the field for this,” Wilson told physicsworld.com.

Slow chemical process

Wilson was quick to point out that the water uptake in rehydroxlation is not the same as absorption — it is a much slower chemical process. The researchers established that the rate at which ceramic materials gain extra water in this process obeys a (time)1/4 power law. They calculated that the rate of reaction is independent of atmospheric moisture levels but is governed by the ambient temperature averaged over a ceramic’s lifetime.

The dating procedure involves measuring the mass of a sample of ceramic and then heating it to around 500 degrees Celsius in a furnace, which removes the water. The re-fired ceramic is then weighed immediately, using a highly accurate microbalance, to determine precisely the rate of water recombination. Once the rate is known, the age of the artefact can be extrapolated. “There are no loose ends with this — everything ties in,” said Wilson.

The researchers dated a Roman brick, known to be 2001, as 2000 years old. They also tested a “mystery brick”, with the real age revealed to them only after their testing was completed — they got 340 years, and its known age was 339 to 344. An interesting thing occurred when they tested their technique on a medieval brick from Canterbury: after repeated testing dated it at 66 years, they realized that the intense heat generated during a Second World War blitz had re-fired the brick and effectively reset its clock.

Self-calibrating

At present, the most widely used alternative technique is thermoluminescence, which involves measuring the amount of light given off by a sample because this is related to the dose of radiation an artefact has received across its lifetime. One of the limitations is that it requires a lot of extra information about the archaeological site such as radiation levels, which may not be accessible if artefacts have already been sitting in a museum for many years. Perhaps the most significant feature of this new water-based technique is that — as with radiocarbon dating — it is self-calibrating, based on rehydroxylation alone.

“The time-dependent processes that they have studied looks very interesting,” said Ian Bailiff, an archaeologist at Durham University in the UK. However, Bailiff is a bit sceptical about the reliability of historic temperature records. “The devil is likely to be in the detail - the chronometric mechanism is temperature dependent, and much work may need to be done to obtain calibration data.”

The researchers are now planning to test whether their dating technique can be applied to earthenware, bone china and porcelain. Wilson told physicsworld.com that one of the main difficulties so far has been getting access to ceramics from museums and collectors who are yet to be convinced by the new technique. She believes this situation will improve if her team can establish an international research centre in the UK — she is currently looking into ways of achieving this.

7 comments

RATE OF RECOMBINATION WITH WATER IS KNOWN?

I look forward to hearing more about the utility of this dating technique.

The article states that in 2003 it was discovered that the rate of recombination of fired clay ceramics with water "has occurred at a predictable rate throughout history, related to temperatures."

I assume that the rate of recombination also depends on humidity.

How much error is introduced by cyclic changes in Earth's climate, e.g., the Little Ice Age of ~1650-1850, and traumatic local events like volcanic eruptions, fires, wars, and crater-producing impacts?

The rehydroxylation rate constant α is insensitive to water vapour pressure at all normal environmental levels (Wilson et al. 2003). This is because the long-term uptake of water is both small and slow so that the water demand is extremely modest; and because the rehydroxylation rate is controlled entirely by internal processes and does notincrease when water is available in excess in the microenvironment. However, the rate does increase markedly with temperature.

The rehydroxylation rate constant α is insensitive to water vapour pressure at all normal environmental levels (Wilson et al. 2003). This is because the long-term uptake of water is both small and slow so that the water demand is extremely modest; and because the rehydroxylation rate is controlled entirely by internal processes and does notincrease when water is available in excess in the microenvironment. However, the rate does increase markedly with temperature.

Indeed, the original research that first identified the power law, showed that the process was relatively indifferent to environmental conditions. Quoting from their article in Phys. Rev. Lett. 90, 125503 (2003)

"Some exploratory experiments show that the early timeexpansions are rather insensitive to the environmentalconditions provided only that some water is available."

The authors then go on to show the results of these exploratory experiments (see figure 2 in the above referenced paper).

Pottery gets refired lots of times during use, and sometimes from wildfire at much later dates. How is that accounted?

That is of course the problem. The researchers found that if the ceramics were heated above 450 C then the clock would be effectively reset. This was demonstrated with bricks from Canterbury, which were known to be from the medieval period, but repeatedly gave ages of 66 years when tested. The researchers were rather confused by this anomaly, until further research revealed that the bricks were victims of World War 2 incendiary bombing in 1942. the intense heat effectively reset the clock on these samples.